1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus provided with separate recording and reproducing light sources, which performs recording and/or reproduction of information with respective light beams being radiated from the respective light sources toward an optical information recording medium.
2. Related Background Art
Various optical information recording media are heretofore known for recording and/or reproduction of information with light, for example, those of a disk, a card, a tape, and the like. Some of such optical information recording media may be used both for record and for reproduction, while others may be used only for reproduction. In a case that information is recorded in a recordable medium, an information track is scanned with a light beam modulated according to recording information and converged in the form of a micro spot so that the information may be recorded as an optically detectable information pit string.
When the information is reproduced from the recording medium, the information pit string in the information track is scanned with a light beam spot having a constant power with which no record can be effected on the recording medium, and light reflected from or transmitted through the medium is detected to reproduce the information.
An optical head, which is used for recording and/or for reproduction of information in the recording medium, is arranged movable relative to the recording medium in a direction of the information track and in a direction across the information track direction. The movement of the optical head permits the scanning of an information track with the light beam spot. To converge the light beam spot in the optical head, an objective lens is used, for example. The objective lens is held to be movable independent of the optical head body in a direction of the optical axis of the objective lens (in the focusing direction) and in a direction perpendicular to the optical axis and to the information track direction of the recording medium (in the tracking direction). The objective lens is usually held through an elastic member and is driven to move in the above two directions by an actuator using the magnetic interaction.
Incidentally, among the aforementioned optical information recording media, the optical information recording medium of a card type (as will be referred to as an optical card) has a great future demand as a prospect for an information recording medium having a relatively large capacity and being compact and light, so as to be handy to carry.
FIG. 1 is a schematic plan view of a writing once optical card, and FIG. 2 is an enlarged view of a part of the optical card.
In FIG. 1, a plurality of information tracks 2 are arranged in parallel with the direction of L-F on an information recording surface of an optical card 1. A home position 3 is provided on the information recording surface of the optical card 1 as an access reference position to the information tracks 2. The information tracks 2 are arranged in the order of 2-1, 2-2, 2-3, . . . from the nearest to the home position 3. As shown in FIG. 2, tracking tracks are provided adjacent to the respective information tracks in the order of 4-1, 4-2, 4-3, . . . The tracking tracks 4 are used as a guide for autotracking (as will be hereinafter referred to as AT) in which the beam spot is controlled so as not to deviate from a certain information track during scanning with the light beam spot in information recording and/or reproduction.
The AT servo is carried out as follows: A deviation (AT error) of the light beam spot from the information track is detected in the optical head; the detected signal is subject to negative feedback to a tracking actuator; and the objective lens is moved relative to the optical head body in the tracking direction (direction of D) to make the light beam spot follow the desired information track.
Also, an autofocusing (as will be referred to as AF) servo is carried out to obtain a spot of the light beam in an appropriate size (that is, to make the light beam focused on the optical card surface) on the surface of the optical card during scanning of the information track with the light beam spot in information recording and/or reproduction. The AF servo is performed as follows: A deviation (AF error) of the light beam spot from an in-focus state is detected in the optical head; the detected signal is subject to negative feedback to a focusing actuator; and the objective lens is moved relative to the optical head body in the focusing direction to make the light beam spot focused on the optical card surface.
In FIG. 2, S1, S2, and S3 represent light beam spots. The tracking is conducted using the light spots of S1 and S3, while the focusing and production of information pits in recording, and the reading of the information pits in reproduction are done using the optical spot of S2. In the respective information tracks, 6-1, 6-2, and, 7-1, 7-2 respectively represent left address portions and right address portions preformatted, which are read for identification of the tracks. Numeral 5 designates a data portion, which is numbered as 5-1 or 5-2 in FIG. 2 and in which predetermined information is recorded.
The method of optical information recording is briefly explained here. Roughly classified, the optical information recording methods are of two types. One is a one light source type in which recording and reproduction are carried out with a common light source, and the other is a two light source type in which recording and reproduction are carried out with two different light sources. It is commonly understood that the two light source type is advantageous with respect to degradation of reproducing light and with respect to an increase of process speed, as compared to the one light source type.
FIG. 3 is a schematic drawing of an optical system in an optical head of the two light source type. The two light source type enables prevention of the reproducing light degradation and the high speed recording with provision of separate light sources for recording light and for reproducing light.
In FIG. 3, reference numerals 21, 22 denote semiconductor lasers as light sources. The semiconductor laser 21 emits light with a wavelength of 780 nm, and the semiconductor laser 22 emits light with a wavelength of 830 nm. Numerals 23, 24 designate collimator lenses, 25 a diffraction grating for splitting a light beam, 26 a dichroic prism which transmits P-polarized light of 780 nm and reflects P-polarized light of 830 nm, 27 a beam shaping prism, and 28 a polarization beam splitter. Also, numeral 29 represents a quarter wave plate, 30 an objective lens, 31 a band pass filter which transmits only the light of 780 nm, 32 a stopper, 33 a toric lens, and 34 a photo detector.
Light beams emitted from the semiconductor lasers 21, 22 enter the collimator lenses 23, 24 in the form of a diverging beam to be modified into collimated light beams, respectively. The light of 780 nm then enters the diffraction grating 25 to be split into three effective light beams (a zeroth order diffracted beam and .+-. first order diffracted beams). The split light beams of 780 nm and the light beam of 830 nm are incident as P-polarized components into a dielectric multilayer film built in a Joint area of the dichroic prism 26 having a spectral property as shown in FIG. 4.
As apparent from FIG. 4, the dichroic prism 26 has such a property that it transmits P-polarized light of 780 nm but reflects P-polarized light of 830 nm. Thus, the light beam of 780 nm is transmitted and the light beam of 830 nm is reflected, so that the light beams are combined with each other to be output as an optical flux from the dichroic prism 26 in the combined state. The optical flux output from the dichroic prism 26 is shaped to have a certain light intensity distribution by the light beam shaping prism 27, and then enters the polarization beam splitter 28.
The polarization beam splitter 28 has such a spectral property as shown in FIG. 5 that it transmits P-polarized light and reflects S-polarized light. The optical flux including the light beams of the two wavelengths is transmitted, because the beams are P-polarized.
Then, the optical flux including the beams of the two wavelengths is changed into an optical flux of circular polarized light when passing through the quarter wave plate 29, and is converged by the objective lens 30. The light beams of 780 nm form three micro beam spots S1 (+ first order diffracted light), S2 (zeroth order diffracted light), and S3 (- first order diffracted light) on the optical card 1, which are used as reproducing light and as signal light for AT and AF controls. The light beam of 830 nm forms a micro beam spot S2 (zeroth order diffracted light) on the optical card 1, which is used as recording light.
Positions of the light beam spots on the optical card 1 are as shown in FIG. 2: The light beam spots S1 and S3 are located on adjacent tracking tracks 4 and the light beam spot S2 is located on an information track 2 between the adjacent tracking tracks. It is preferred as to a positional relation between S2 of 780 nm and S2 of 830 nm that the light beam spot S2 of 830 nm as recording light leads the other in the moving direction. There is, however, no theoretical restriction on the arrangement of the beam spots S2, S2. They are located at the same position in this embodiment. The light beam spots formed on the optical card 1 are reflected to pass through the objective lens 30 to become parallel. The light beams again pass through the quarter wave plate 29 to have a polarization direction rotated by 90 degrees as compared to that upon incidence thereinto. The light beams are incident as S-polarized light beams into the polarization beam splitter 28. Since the splitter 28 reflects S-polarized light as described, the light beams are reflected toward the band pass filter 31. The band-pass filter 31 has such a spectral property that it transmits light near 780 nm as shown in FIG. 6. Thus, the band pass filter 31 transmits the light near 780 nm but reflects light of other wavelengths. The band pass filter 31 guides the light of 780 nm as signals to a detection optical system. The light passing through the band pass filter 31 is converged by the toric lens 31 to enter the photo detector 34. The photo detector 34 is constructed as shown in FIG. 7, which carries out the tracking control with signals received by light receiving elements 11, 13, and the focusing control and the reproduction signal detection with a signal received by a light receiving element 12 which is divided into four sections.
In the example of the optical head as shown in FIG. 3, however, the reproducing light and the recording light are radiated from the respective light sources and a positional deviation is inevitably caused between the reproducing beam spot and the recording beam spot, which results in degradation of reproduction signals.
The positional deviation could be adjusted by moving the light sources to prevent the degradation of reproduction signals. Supposing the focal length of the collimator lens is equivalent to that of the objective lens, which is the case in common optical disk apparatuses, the light source is to be moved by 0.1 .mu.m for adjustment of position deviation of 0.1 .mu.m. If the accuracy of submicron order is required, the time and the cost for adjustment would be extremely increased.